Embodiments of the present invention relate to test patterns, and more particularly relate to test patterns and associated techniques for testing the fidelity of intensity reproduction in image output systems.
Conventional display devices reproduce encoded intensity values in a nonlinear fashion. For example, CRTs (Cathode Ray Tubes) exhibit a voltage (V) to intensity (I) response that corresponds to a nonlinear transfer function of the form I∝Vγ, where the exponent γ (gamma) is approximately 2.2. This phenomenon results in an apparent darkening of the mid-tones in a displayed image.
To compensate for the above effect, an inverse transfer function may be applied to a signal prior to the point of display/output. The application of this inverse transfer function, known as gamma correction, is designed to cancel out the nonlinearity introduced by the display device, and thereby produce an end-to-end (i.e., source to output) response that is approximately linear. In order to accurately control the tone scale of a displayed image such that said image accurately corresponds to the original signal, the gamma of the display/output must be accurately known. Precise gamma calibration thus plays an important role in achieving high fidelity image reproduction.
Many image output devices (e.g., televisions, computer monitors, etc.) and image output driver devices (e.g., DVD players, Blu-Ray players, HD-DVD players, digital video recorders, computers, etc.) have a user control labeled “gamma” that alters the exponent of the transfer function applied at the display device or the inverse transfer function applied through gamma correction. Thus, this control affects the end-to-end gamma response of an image output system or chain. Many devices without such a labeled control still provide other controls which directly or indirectly affect end-to-end gamma response.
A common type of test pattern that is used to aid in the visual adjustment of gamma comprises an area of alternating white (100% intensity) and black (0% intensity) and an area of continuous gray (e.g., 50% intensity). This pattern relies on the low-pass filtering effects of distance on the human visual system to “fuse,” in the mind of the viewer, the area of white/black into a single intermediate intensity. This intermediate intensity is then compared to the area of continuous gray to determine whether the intensity of the continuous gray is correct.
One problem with the above pattern is that the comparison of the area of white/black with the area of continuous gray only tests, or samples, a single point on a gamma curve. While multiple pairs of white/black and grey areas may be incorporated to test different intermediate intensities on a single pattern, the number of sample points will still be relatively limited. Additionally, certain display types reproduce varying intensities for white and black depending on the amount of white and black in the displayed image. Making such comparisons may also be difficult for some viewers, especially for large-screen displays. As a result, the pattern described above may not allow for an accurate adjustment of gamma for all types of images/scenes.